It is well known to use, for producing oxygen with low energy, a double air separation column which is applied, in particular, on the one hand, so as to minimize the delivery pressure of the air compressor, by reducing the head losses in the exchanger and reducing the temperature difference at the main vaporizer, and, on the other hand, to maximize the oxygen extraction efficiency, by reducing the temperature difference in the exchanger, by choosing a high number of theoretical distillation trays and by installing a sufficient number of sections of structured packings or trays.
Thus, low-pressure columns have four sections of structured packings or trays, including two sections between the bottom of the low-pressure column and an intake for rich liquid, this being an oxygen-enriched liquid taken from the bottom of the medium-pressure column. These two sections are necessary for providing high-performance distillation in the bottom of the low-pressure column. Thus also, the medium-pressure columns have four sections of structured packings or of trays, including two sections between the liquid air intake and the point of withdrawal of lean liquid.
The exchanger of an air separation unit is normally composed of an exchange body assembly or of several body subassemblies.
An exchange body assembly comprises an even number of exchange bodies, each of which is fed with the same fluids to be cooled and the same fluids to be warmed. The fluid feed is made via a common header line for each different fluid (different composition and/or pressure), as illustrated in FIGS. 1-3 of “The Standards of the Brazed Aluminum Plate-Fin Heat Exchanger Manufacturers ' Association”, 2nd Edition, 2000.
Since the maximum number of bodies that can be fed via a single header line is 12 (i.e. 6 pairs of exchange bodies), it is often necessary for large-capacity units to use several exchange body subassemblies, each subassembly comprising an even number of exchange bodies and the bodies of each subassembly being fed via a common header line for each different fluid. Thus, an exchanger composed of two exchange body subassemblies will comprise a first delivery line sending air to be cooled to the first subassembly and a second delivery line sending air to be cooled to the second subassembly. Likewise, it will comprise a first header line recovering the cooled air from the first subassembly and a second header line recovering the cooled air from the second subassembly.
The purified and compressed air sent to the columns cools in an exchanger comprising a single body assembly which would normally have a volume of more than 200 m3, and therefore with a ratio of the total air volume sent to the exchanger to the volume of the exchanger that would be approximately 2,000 Nm3 /h/m3 in the case of the example described below.
The refrigeration required for the distillation is frequently provided by an air stream sent to a blowing turbine that feeds the low-pressure column and/or an air stream sent to a Claude turbine. The ratio of the quantity of air sent to the exchanger to the volume sent to the blowing turbine would normally be between 5/1 and 15/1 in the case of the example described below.
In certain cases when energy is not expensive, or even free, it is profitable to reduce expenditure on equipment, while increasing energy requirements.
In a process for separating air by cryogenic distillation known from WO 03/033978 using an apparatus comprising a medium-pressure column and a low-pressure column that are thermally coupled, a quantity of compressed and purified air V is cooled in an exchange line down to a cryogenic temperature and is sent at least partly to the medium-pressure column, oxygen-enriched and nitrogen-enriched streams are sent from the medium-pressure column to the low-pressure column and nitrogen-enriched and oxygen-enriched streams are withdrawn from the low-pressure column, the medium-pressure column operating between 6 and 9 bar absolute and the ratio of the volumetric flow rate of air V entering the exchanger to the total volume of the exchanger being between 3,000 and 6,000 Nm3/h/m3.
With a ratio of the volumetric flow rate of air V entering the exchanger to the total volume of the exchanger of less than 6,000 Nm3/h/m3, and by considering an air separation unit having a volumetric flow rate of air of about 570,000 Nm3/h, the total volume of the exchanger is about 110 m3 with an exchanger composed of at least 14 exchange bodies, the maximum volume of an exchange body being about 8 m3.
As regards questions about the uniform distribution of the streams between the various exchanger bodies, the prior art dictates two exchange body subassemblies, a first subassembly of which comprising 8 exchanger bodies grouped together in four pairs and a second subassembly of which comprising six exchanger bodies grouped together in three pairs. It is not conceivable to install a single assembly of 14 exchanger bodies (the distribution of the streams will not be uniform because of the long distances that exist in this case between the bodies, and the performance of the air separation unit will be affected).
With a ratio of the volumetric flow rate of air V entering the exchanger to the total volume of the exchanger of about 7,000 Nm3/h/m3, and considering an air separation unit having a volumetric flow rate of air of about 570,000 Nm3/h, the total volume of the exchanger is about 80 m3 with a single exchange body assembly that is composed of 10 exchanger bodies, the maximum volume of an exchange body being about 8 m3. In this case, the uniform distribution of the streams between the various exchanger bodies is achieved favorably with a single exchange body assembly, so that there is only a single common delivery or header line for each fluid fed into or coming from the 10 bodies.
Likewise, for an air separation unit having a volumetric flow rate of air of about 475,000 Nm3/h, owing to the low cost of the energy or to the amount of energy available, the investment cost will be minimized by installing an exchange line composed of a single assembly of exchanger bodies (8 bodies) and the volume of which will correspond to a ratio of the volumetric flow rate of air V entering the exchanger to the total volume of the exchanger of about 7,400 Nm3/h/m3.
Moreover, increasing the ratio of the volumetric flow rate of air V entering the exchanger to the total volume of the exchanger ought to result, according to the prior art, in an increase in the head losses in the exchanger for all the streams of the exchanger (waste nitrogen stream, air streams, oxygen stream, etc.), especially because of the increase in the flow rate due to the reduction in flow area.
However, for ratios of the volumetric flow rate of air V entering the exchanger to the total volume of the exchanger greater than 6,000 Nm3/h/m3, the head losses on the oxygen stream will not be increased but will be constant at a limiting value corresponding to a usually acceptable design with regard to an oxygen stream. To keep the oxygen stream rate constant while reducing the volume of the exchanger is generally possible only by keeping a constant flow area for each body of the exchanger, and therefore keeping the total number of passages of the exchanger with regard to the oxygen stream constant, which results in an increase in the number of oxygen passages in each body of the exchanger (since the number of bodies of the exchanger is reduced). Consequently, the head losses on the other streams will therefore increase more than what is obtained by the simple ratio of the number of bodies.
However, in particular in the case of liquid oxygen passages in which the liquid has to vaporize, a variable flow area, or an increase in the flow area, may be provided.
Typically, the head losses with regard to the oxygen stream will not exceed 400 mbar and the flow area with regard to the oxygen stream will not exceed 20 to 25 Nm3/h/cm2. The flow area corresponds either to the constant cross section or to the cross section at the point where the liquid vaporizes, for the case of a liquid stream.
The oxygen stream comprises at least 30 mol % oxygen, preferably at least 70 mol % oxygen, and even more preferably at least 90 mol % oxygen, and may be in gaseous or liquid form at the inlet of the exchanger.